Efficiencies of photovoltaic cells, in particular next-generation technologies such as organic photovoltaic cells (OPV), must be increased to achieve commercial viability in expanding applications. Specifically, efficiency in technologies such as OPVs is often limited by weak light absorption in the active layer. To compensate, active layer thickness is increased. However, increasing the active layer thickness results in fewer charge carriers successfully reaching their respective electrodes and ultimately the external load. In OPVs, in particular, the polymers and other organic semiconductors that are used in the active layer have charge carriers with mobilities that are lower than traditional inorganic counterparts.
Rather than increasing the active layer thickness, light absorption can be increased by increasing the effective path length for photons through light trapping. Instead of traveling through the solar cell unabsorbed, a reflective electrode reflects light that enters the solar cell and allows it additional opportunities to be absorbed by the solar cell. Employing static structures in the reflective electrode, Surface Relief Gratings (SRGs), further enhances light absorption. Periodic, sub-micrometer substrate structures can trap light within the active layer by reflecting it at higher angles back through the film rather than letting it reflect straight out of the device as with a flat substrate. SRG's can also be used with non-OPVs such as crystalline silicon photovoltaic cells.
However, the geometry of the SRG must be optimized for a given set of conditions, including the angle of light incidence. A solar module experiences a broad range of incident angles as the sun traverses the sky during the day; moreover, clouds, precipitation, and seasonal changes can also affect the ambient light environment. Accordingly if the SRG is static, optimal conditions are only met for one instant on one day per year. Therefore a need exists in the industry to provide solar cell systems with SRGs that adapt to changing light conditions.